1. Field of the Invention
The present invention relates in general to a method for forming a pattern during the manufacturing of a semiconductor. More particularly, it relates to a method for solving the T-top formation problem in a chemically amplified resist process.
2. Description of the Related Arts
Chemically amplified resists are an important class of resists for deep UV exhibiting high photo-speed, excellent resolution, and process tolerance. The chemically amplified resist consists of a polymer host and a generator of acid in the presence of light. In the post-exposure bake, the photogenerated acid catalyzes thermal reactions that alter the solubility of the exposed region. Generally, resolution and sensitivity are affected by post-exposure conditions.
Prior art chemically amplified positive photoresists, however, suffer from the problem known as post-exposure delay (PED). When deep-UV lithography is carried out, line patterns have a T-top configuration; that is, patterns become thick at the top if the delay time from exposure to post-exposure baking is extended. This problem not only makes dimensional control in the lithography process difficult, but also adversely affects dimensional control in the processing of substrates using dry etching.
It is understood that basic compounds in the air largely cause in the T-top formation phenomenon for these chemically amplified positive resist materials. In the following, a conventional pattern forming method will be described with reference to FIGS. 1A to 1D, by which the T-top formation phenomenon will be illustrated.
Referring to FIG. 1A, a semiconductor substrate 1 is coated at its top with the chemically amplified resist, thereby forming a resist film 2 with a thickness of 1 .mu.m. Next, as shown in FIG. 1B, exposure 4 is performed at 25 mJ/cm.sup.2 on the resist film 2 through a mask 3, using a KrF excimer laser stepper, whereby acid is generated in the exposed regions.
Next, as shown in FIG. 1C, after performing heating 5 for 90 seconds on the semiconductor substrate 1 at a temperature of 95.degree. C., the exposed regions of the resist film become alkali-soluble under the catalysis of the photogenerated acid. After this, the resist film 2 is developed for 60 seconds in an alkaline solution of 2.38 wt %, whereby a resist pattern 6 of a positive type is defined as shown in FIG. 1D.
However, in the resist pattern 6 which is formed, acid which is generated by the acid generating agent is easily deactivated by basic compounds in the air since it is only present in a catalytic amount. Thereby, the profile of the resist pattern 6 often deteriorates, as shown in FIG. 1D. In short, the photogenerated acid is deactivated due to a basic compound, such as NH.sub.3 or NMP, which usually exists within the clean room. Thereby, an upper portion of the resist pattern 6 does not become alkali-soluble, whereby an upper portion of the exposed portion of the resist pattern 6 remains even after developed in an alkaline solution.
This T-top formation of the resist film will result in pattern profile change and CD variation when etching is performed on a film using such a resist pattern.
There is a strong demand for a method for solving the T-top formation problem of chemically amplified positive resists. Examples of prior art methods regarding this can be found in U.S. Pat. Nos. 5,756,262 to Endo et al. and 5,629,134 to Oikawa et al.